1. Field of the invention
This invention relates to methods and apparatus for flanging tubular polymer articles, and more specifically to the flanging of the open ends of tubular articles of crystallisable thermoplastic polymers, particularly of saturated linear polyester materials such as polyethylene terephthalate. The tubular articles may be open at both ends or may have one open end and one closed end (integral or otherwise), and they may, for example, be used as bodies for processable food and beverage containers, in which case the or each flange will usually (but not necessarily) be provided for the purpose of being seamed to an end component, e.g. of metal. The tubular bodies may be of circular, rectangular or other cross-section.
2. Description of the Prior Art
Techniques of flanging metal can bodies preparatory to attaching metal end components by seaming are well known. The flange is essentially cold swaged, using a number of rollers on a rotating head. If this method is applied to a body of a crystallisable thermoplastic polymer, the body is simply cut without any flange being formed. It is known to flange bodies made of fibre composite board for forming containers by pressing the end of the tubular body axially into a die, which is usually cold although it is known for it to be heated. So long as the moisture content of the fibre composite board is correct, a well defined flange can be rapidly produced. If this method is used with a tubular body of crystallisable thermoplastic polymer, however, the flange substantially wholly recovers the original shape of the tubular body once the axial force has been released.
It is known that biaxially drawn crystallisable thermoplastic polymers normally tend to shrink if heated above the temperature at which they were drawn, but that this tendency can be much reduced by annealing the material under restraint, e.g. at temperatures in the range of 150.degree. C. to 230.degree. C. This process is known as heat setting. It usually leaves the material with a residual linear shrinkage of up to about 3% which will appear on re-heating without restraint to the drawing temperature, but the heat-set articles are otherwise dimensionally stable and strong.
An object of the present invention is to provide a method and apparatus by which a well defined flange can be reliably formed upon the end of a tubular article of a heat-set crystallisable thermoplastic polymer, e.g. a saturated linear polyester such as polyethylene terephthalate. A particular object is to enable the formation of such flanges on the open ends of tubular articles which are to form the bodies of containers, which are subsequently to be seamed to metal end components.